Slurry trenching methods for forming narrow underground walls has been extensively developed in recent years by the assignee hereof and is now widely used all around the world (See the text "The I.C.O.S. Company in Underground Works" Vol. 3, I.C.O.S. Milan 1968 and also see the text by R. G. H. Boyce entitled "Structural and Cut-off Diaphragm Walls"; John Wiley and Sons (1978). In this technology, the slurry, frequently a mixture of sodium bentonite and water and possibly other chemicals, permits open trench excavation to be carried out without mechanical bracing or shoring-up of the walls because the slurry is heavier than water and develops a pressure in the trench which in conjunction with a bentonite cake supports the side walls of the trench until the wall forming material is inserted in the trench. Frequently, the wall forming material is a mixture of bentonite slurry and excavated soil which has been prepared to have a certain consistency in a backfill operation or step. Thus, a trenching machine can be used to move along the line of the wall and as it excavates soil material, the soil can mix with the bentonite in the slurry, or from a separate source, and with or without further processing, the excavated material can be backfilled into the core of the trench. Sometimes, the excavated material is laid to a special area beside the trench where various machines are used to reduce the backfill to the proper consistency and then it is simply bulldozed into the trench. In other cases, the backfill material is processed through shakers screens and the like which are moved along the side of the trench. At any rate, the heavy equipment used for excavating the trench, processing the slurry and preparing the backfill can sometimes overload the sides of the excavation and hence require a very large amount of working space and room.
The object of the present invention is to provide an excavating apparatus and method which floats on the slurry in the trench. A further object of the invention is to float at least a portion of the apparatus on the slurry and provide in one single operation the excavation of deep slurry trenchs, mixing and the placing of the backfill. A still further object of the invention is to utilize the slurry in the trench to support all or part of the wall forming apparatus.
A further object of the invention is to reduce the equipment working space required along the line of the wall and to make the equipment narrow. This is particularly significant when a working platform (the space along the line of the wall that is leveled and/or stripped for performance of the work) of a minimum width has to be created to perform the work by building an embankment (on a dike for example) or cutting a trough (in uneven terrain for example) along the line of the wall and which, by its nature has to have a minimum slope.
While the invention is directed to making underground walls, it will be appreciated that the invention cal also be used for laying pipe, utility lines, cables etc.
According to the present invention, these functions can be performed automatically for the construction of a cut-off wall without overloading the sides of the trench and also will provide automatically alignment of the backfill into the trench itself. The apparatus is comprised substantially of four parts:
a floatation device, PA1 a mixing and placing unit, PA1 a cutting head and PA1 a power pack.
The floatation device can be of several different forms. By stacking floats in modular form, several tons of floatation can be achieved even in a very narrow trench. The mixing apparatus as well as the conveyor belt delivering the backfill are floating in a preferred embodiment and do not vertically load the sides of the trench. As opposed to what occurs when the backfill is placed by a bulldozer, the stability of the trench will not be jeopardized by vibrations. The modular float system can be easily transported and adapted to various job conditions. Finally, by introducing into the floats more or less liquid, which, in the preferred embodiment, will be of same composition as the slurry in the trench, greater or less floatation can be achieved so as to increase traction on low pressure wheels and/or tractor crawler treads so that the loading on the side walls can be very precisely controlled.
When the wall is to be a mixture of soil and bentonite, the mixing and placing unit is preferably a floating unit which will be stabilized by low pressure tires which will be just barely loaded and will not create substantial pressure on the sides of the trench. As indicated above, the loading on the tires or the caterpillar treads can be adjusted by partial flooding of the float, if this be desired. This floating unit can also act as a platform for the measurement to the bottom of the trench as well as for accomodating rock removing devices and/or for creating a key in the bottom of the trench. By regulating the input of the air lift, the speed of the vibrating drum, the backfill mix composition can be easily adjusted to suit any consistency required. Since the backfill material is, in a preferred embodiment, constituted by excavated soil mixed with the bentonite slurry from the trench, the mixing unit can be a vibrating drum or screen into one end of which the excavated soil material is introduced, the slurry is returned to the trench directly below the vibrating drum and a mixture of soil and bentonite is delivered from the opposite side to the floating conveyor to be delivered to the opposite end of the trench from the excavating end.
The system allows for easy addition to the backfill material at the mixing point and for changes in the grain size distribution of the backfill. The backfill can be given added strength or can be modified with the addition of chemical additives, cements and the like at the mixing point, if this is desired. Since the typical yield of worked excavated material exceeds the volume of soil in place by some 20 to 30% (where this is the kind of wall being constructed) this additional material can be deposited as a continuous cap over the trench which results in the following additional advantages: (a) the requirement of a clay cap which is almost always present in slurry trench construction is automatically satisfied, (b) excess material will fill any settlements of the backfill and (c) when no longer required can be easily removed at a later date but it does not require additional working area.
The cutting head can be any of a variety types powered by submersible motors driven by hydraulic, electric and/or mud pumps. Depending upon the various types of cutting head configurations, they can reach any practical depth since the cutting head can be weighted down at the bottom and connected to the mixing unit by a flexible hose which will carry the soils up with an air lift. Moreover, the cutting head can be changed easily to operate in different types soils and the cutting head can also operate vertically and be part of the mixing and placing unit. Moreover, the cutting head can be a screw conveyor operated in a reverse direction to convey the excavated soil material in a downward direction to an air lift located at the bottom of the trench so that the bottom of the trench will always be clean as the excavation proceeds.
The power pack can move ahead of the trenching operation, if desired, without loading the sides of the excavation and acting as a tractor. The power pack contains air, electric or hydraulic power as may be required. If the use of an air lift in the cutting head will necessitate a pretrenching to a selected minimum low depth, such pretrenching operation can be performed by the power pack unit if desired. Because of the flexible configuration of the machine in plan, it can follow curve contours. Due to its fully automatic operation, it minimizes the use of personel, especially a non-union areas. It is composed of easily replaced parts and therefore it is highly efficient. The combination of the power pack and mixing and placing unit can be consolidated in one apparatus which will excavate a vertical face of the trench. Due to the use of air lift, the machine automatically cleans and descends to the bottom of the excavation as it progresses, eliminating this separate operation which is typically required in slurry trench construction. Due to the geometry of the cutting head it can be easily adapted to very irregular contours of the bottom of the trench and the cutting head and the mixing unit can easily accomodate a walking beam system operating a chisel at the bottom to provide for keying the slurry trench into the underlying rock formation if desired.